1. Field of the Invention
The present invention relates to a spark plug used in an internal combustion engine, and more particularly to a resistor spark plug including a resistor for preventing generation of radio noise.
2. Description of the Related Art
An existing resistor spark plug has the following structure. A terminal metal piece is inserted into one end of an axially extending through-hole of an insulator and is fixed thereto. A center electrode is inserted into the through-hole from the other end and is fixed thereto. A resistor is disposed within the through-hole to be located between the terminal metal piece and the center electrode. The resistor is formed of a oil mixture of glass and a conductive material such as carbon black or metal. However, since the metal content of the resistor is not very high, in many cases, direct connection with the terminal metal piece or the center electrode is difficult to achieve. Therefore, in general, a conductive glass seal layer formed of a mixture of glass and a relatively large amount of metal is disposed between the resistor and the terminal metal piece and between the resistor and the center electrode in order to increase the bonding strength.
Such a resistor spark plug is manufactured as follows. After a center electrode is inserted into and fixed to a through-hole of an insulator, powder of conductive glass is charged into the through-hole. Subsequently, powder of resistor composition material is charged into the through-hole, and powder of conductive glass is again charged into the through-hole. Finally, a terminal metal piece is press-fitted into the through-hole from an end opposite the center electrode, to thereby obtain an assembled unit. Thus, within the through-hole of the insulator, a layer of conductive glass powder, a layer of resistor composition powder, and another layer of conductive glass powder are successively layered from the side of the center electrode. The assembled unit is placed in a heating furnace to be heated to a temperature above the melting point of glass. Subsequently, the terminal metal piece is pushed toward the center electrode to compress the respective layers, so that the layers become a conductive glass seal layer on the center electrode side, a resistor, and a conductive glass seal layer on the terminal metal piece side. Thus is completed a structure in which the terminal metal piece and the center electrode are connected to the resistor via the respective conductive glass seal layers.
When a resistor spark plug is manufactured in the above-described manner, during the heating/compressing step, a tip end portion of the terminal metal piece is pushed into a layer of conductive glass powder that has been softened through heating, and finally the terminal metal piece is joined to the conductive glass seal layer in a state in which the tip end portion of the terminal metal piece is located in the conductive glass seal layer. In order to obtain a strong joint, it is important that the clearance between the outer circumferential surface of the tip end portion of the terminal metal piece and the inner surface of the through-hole of the insulator is sufficiently filled with the conductive glass seal layer. However, since the clearance is generally small, and the viscosity of softened conductive glass is not very high, the charge of glass is frequently insufficient. In this case, the bonding or bonding strength between the terminal metal piece and the conductive glass seal layer is insufficient, with a resultant possibility of the terminal metal piece coming off upon receipt of an impact. Further, the bonding strength between the terminal metal piece and the conductive glass seal layer easily deteriorates upon repeated application of high voltage to the spark plug.
In order to increase the bonding strength between the terminal metal piece and the conductive glass seal layer to thereby solve the above-described problem, in a generally used spark plug, a thread or knurl is formed on the outer circumferential surface of the tip end of the terminal metal piece, which is to be inserted into the conductive glass seal layer, to thereby increase the bonding strength between the terminal metal piece and the conductive glass seal layer by means of an anchor effect. However, when such a thread or knurl is formed on the outer circumferential surface of the tip end of the terminal metal piece, the charging of conductive glass into the clearance between the terminal metal piece and the insulator becomes more difficult, so that in some cases, the bonding strength, rather than being increased, is decreased.